1. Field of the Invention
The invention relates to a method of fabricating a semiconductor device including an insulating film composed of carbon family material, and more particularly to a method of fabricating a semiconductor device having a multi-layered wiring structure including an interlayer insulating film composed of carbon family material.
2. Description of the Related Art
With an increase in integration of a semiconductor integrated circuit such as LSI, it becomes more and more important to fabricate a multi-layered wiring structure with high reliability in fabrication of a semiconductor integrated circuit.
Reliability of a multi-layered wiring structure is dependent in particular on a characteristic of an interlayer insulating film. A material of which an interlayer insulating film is composed and a method by which an interlayer insulating film is formed have been determined from the standpoint of heat-resistance, electrical insulation, and coverage ability. For instance, an interlayer insulating film has been usually composed of silicon dioxide (SiO.sub.2), silicon nitride (SiN) or phospho silicate glass (PSG), and has been usually formed by chemical vapor deposition (CVD).
However, with a design rule for LSI getting smaller, wiring delay caused by parasitic capacity between wirings has recently become more serious than signal delay caused by individual elements such as a transistor constituting LSI. As a result, due to writing delay, LSI could not be improved, even if individual elements were fabricated in a size as small as possible.
Under such circumstance, there is presently developed an interlayer insulating film having a smaller dielectric constant .di-elect cons.r than a dielectric constant of a silicon nitride film and a dielectric constant of a silicon dioxide film both of which have been widely used as an interlayer insulating film in fabrication of a semiconductor integrated circuit. Herein, both a silicon nitride film and a silicon dioxide film have a dielectric constant .di-elect cons. r smaller than seven (7).
An insulating film composed of carbon family material attracts attention as a material having a small dielectric constant to be used in place of an insulating film composed of silicon family material, because an insulating film composed of carbon family material is expected to have a smaller dielectric constant. Herein, carbon family material of which an insulating film is composed includes organic polymer family material such as polyimide, poly-p-xylylene, and benzocyclobuten, and inorganic carbon family material such as amorphous carbon.
An insulating film composed of such carbon family material has a smaller dielectric constant than that of a conventional insulating film composed of silicon family material, and could have a further smaller dielectric constant by additionally containing fluorine thereinto. For instance, if an amorphous carbon film contains fluorine therein, it would be possible to decrease a dielectric constant of the amorphous carbon down to approximately 2. Composing a semiconductor device of such an amorphous carbon film has recently been reported.
The inventors had reported a method of fabricating a multi-layered wiring structure including an interlayer insulating film composed of fluorinated amorphous carbon in IEDM Technical Digest, 1996, pp. 369-372. In this report, combination of a film composed of carbon family material and a silicon dioxide film was suggested in order to make it possible to employ conventional interlayer technology.
The suggested method has the following steps.
First, a lower wiring layer composed of aluminum is covered with an amorphous carbon film containing fluorine therein and having a dielectric constant of 2.3. Then, a silicon dioxide (SiO.sub.2) film is deposited on the amorphous carbon film by a thickness of about 2 microns by plasma-enhanced chemical vapor deposition. The silicon dioxide film is deposited for the purpose of ensuring processability in planarization to be carried out later by chemical mechanical polishing (CMP), and enhancing accuracy in planarization.
When the silicon dioxide film is deposited on the amorphous carbon film, the silicon film is made silicon-richer at least at an interface between the silicon dioxide film and the amorphous carbon film in order to enhance adhesion between the films.
After the deposition of the silicon dioxide film, the silicon dioxide film is planarized by CMP. Then, the amorphous carbon film is formed with via-holes with the silicon dioxide film being used as a hard mask. Then, the thus formed via-holes are filled with aluminum plug. Thus, there is completed a multi-layered wiring structure.
The multi-layered wiring structure formed in the above-mentioned manner can have a parasitic capacity which is reduced by about 50% relative to that of a conventional multi-layered wiring structure including an interlayer insulating film composed of silicon dioxide.
As having been explained so far, in order to form a multi-layered wiring structure including an insulating film composed of carbon family material, with high processability being maintained, it is necessary to deposit an insulating film which is to be planarized later and which is composed of a conventional material such as silicon dioxide, on an interlayer insulating film composed of carbon family material with high adhesion being maintained therebetween.
In addition, a thin film composed of refractory metal such as titanium nitride, titanium, tantalum nitride, and tantalum may be sandwiched between the interlayer insulating film and the silicon dioxide film in order to prevent mutual diffusion between the interlayer insulating film and metal of which a wiring layer is composed, such as aluminum and copper.
As explained above, it is necessary to carry out various steps including steps of depositing films after deposition of an interlayer insulating film composed of carbon family material, in order to fabricate a multi-layered wiring structure. These steps are inevitably accompanied with an annealing step. Hence, it is necessary to prevent various defects such as peeling-off and mutual diffusion from being caused by an annealing step to be carried out after formation of an insulating film composed of carbon family material, between the insulating film and other films to be deposited on the insulating film such as the above-mentioned film to be planarized and refractory metal film. If defects such as peeling-off and mutual diffusion are caused, a semiconductor device could not work properly, resulting in reduction in reliability and fabrication yield.
However, if an insulating film composed of carbon family material is heated up to about 400 degrees centigrade, gas is discharged out of the insulating film. The gas breaks adhesion between the insulating film and a silicon dioxide film or other films deposited on the insulating film, resulting in film peeling-off in a multi-layered wiring structure. Hence, it is necessary to reduce an amount of gas discharged out of the insulating film to thereby prevent peeling-off of films caused in an annealing step to be carried out after formation of the insulating film.
In order to prevent films from peeling off in an annealing step, it is necessary to reduce an amount of gas desorped out of an insulating film by annealing. If an insulating film composed of carbon family material is annealed after deposition of the insulating film, but prior to deposition of films on the insulating film, gas desorped out of the insulating film is discharged out of a multi-layered wiring structure. Accordingly, it would be possible to prevent films deposited on the insulating film from being peeled off, even if those films are subject to annealing.
In a conventional method, an insulating film composed of carbon family material are subject to an annealing step in a vacuum atmosphere or in an inert gas atmosphere such as nitrogen and argon. The conventional method makes it possible to reduce an amount of gas to be discharged in later steps, but is accompanied with a problem that a dielectric constant of the insulating film is increased in comparison with the dielectric constant before carrying out an annealing step. In other words, a small dielectric constant by which an insulating film composed of carbon family material is characterized is incompatible with processability.
For instance, Japanese Unexamined Patent Publication No. 8-195565has suggested a method of fabricating a multi-layered wiring structure, comprising the steps of forming a wiring layer on a substrate, applying photosensitive polymer on the wiring layer, drying the photosensitive polymer, exposing the photosensitive polymer to a light in a pattern, developing the photosensitive polymer to thereby form via-holes in the photosensitive polymer, curing the photosensitive polymer to thereby form an interlayer insulating film, and forming an upper wiring layer on the insulating film.
Japanese Unexamined Patent Publication No. 8-264648 has suggested a semiconductor device including an interlayer insulating film composed of fluorine-containing amorphous carbon, an electrode, a wiring layer, and a buffer layer sandwiched between the interlayer insulating film and the wiring layer. The buffer layer prevents gas from being discharged out of the interlayer insulating film in an annealing step.
Japanese Unexamined Patent Publication No. 9-237837 has suggested a method of fabricating a multi-layered wiring structure, comprising the steps of forming a first wiring layer on a substrate, patterning the first wiring layer, forming a photosensitive polymer layer on the thus patterned first wiring layer, exposing the photosensitive polymer layer to a light, developing the photosensitive polymer layer to thereby form an interlayer insulating film, and forming a second wiring layer on the insulating film.
However, none of the above-mentioned Publications can overcome the problem mentioned earlier that a dielectric constant of the insulating film becomes greater than before carrying out an annealing step.